Adsorbatschwingungen und Substratoberflächenphononen der wasserstoff- und sauerstoffbedeckten (110)-Oberfläche von Molybdän

Kröger, J. P.

Book/Report

FZJ-2019-03727

Adsorbatschwingungen und Substratoberflächenphononen der wasserstoff- und sauerstoffbedeckten (110)-Oberfläche von Molybdän

Kröger, J. P. (Corresponding author)FZJ*

1998
Forschungszentrum Jülich, Zentralbibliothek, Verlag Jülich

Jülich : Forschungszentrum Jülich, Zentralbibliothek, Verlag, Berichte des Forschungszentrums Jülich 3536, 124 p. (1998)

Please use a persistent id in citations: http://hdl.handle.net/2128/22422

Report No.: Juel-3536

Abstract: The two adsorbate systems H/Mo(110) and 0/Mo(110) have been investigated by means of electron energy loss spectroscopy mainly with respect to adsorbate induced anomalies in the dispersion of substrate surface phonons. The clean surface is characterized by an intense loss peak at 225 cm$^{-1}$ in specular scattering geometry which is due to a dipoleactive surface resonance. The atomically adsorbed hydrogen exhibits three ordered superstructures with increasing coverage at a sample temperature of 110 K: at 0.5 ML a (2x2)2H phase was observed which turns into a (2x2)3H phase at 0.75 ML (until now the former has been believed to be the unique (2x2) superstructure); finally at 1ML the saturated (1x1)H superstructure was observed. The adsorption site is the triply coordinated hollow site for all phases. The (1x1)H phase reveals a damped vibrational mode in the specular spectra: the parallel hydrogen vibration mode exhibits a Fano-like line shape which is caused by a nonadiabatic coupling between the discrete vibrational mode and the electronic continuum of the substrate. For all phases, i. e. clean, (2x2)211, (2x2)3H, and (I x1)H, the dispersion of the Rayleighwave and the longitudinal mode was investigated along the symmetry directions of the surface Brillouin zone. The most striking observation is an anomalous indentation of the frequencies of both the Rayleigh-wave and the longitudinal mode at wave vectors of $\approx$ 0.9$\mathring{A}^{-1}$ along [001] and of $\approx$ 1.2$\mathring{A}^{-1}$ along [1$\overline{1}$2]. Concerning the Rayleigh-wave this result has already been demonstrated by means of inelastic helium atom scattering. As a result of the present investigation the suggested connection between the observed specular loss continuum on saturated (1x1)H/W(110) and the likewise occuring phonon anomalies could be ruled out. Thus the current interpretation of this phenomenon in terms of a giant Kohn anomaly due to Fermi-surface nesting between adsorbate induced surface states is sound. In the case of oxygen it could be shown in agreement with the literature that the initial atomic adsorption (< 0 .25 ML) takes place by means of p(2x2) islands with the adatoms occupying the long bridge. At 0.25 ML the adsorbate layer orders by annealing the sample surface and a p(2x2)O superstructure appears. Specular spectra suggest the simultaneous occupation of the long bridge and the triply coordinated hollow site probably by adatoms at the boundaries of islands. Increasing coverage (> 0.25 ML) leads to more complex superstructures and to a change of the adsorption site from the long bridge towards the triply coordinated hollow site. Angle resolved photoelectron spectroscopy data allow for Fermi-surface nesting between surface states of p(2x2)O/Mo(110) but the dispersion curves of the Rayleigh-wave and the longitudinal mode do not reveal any anomalous behaviour. As a result one must conclude that the electronic instability in terms of Fermi-surface nesting is necessary but not sufficient for a giant Kohn anomaly. At least two additional conditions must be fulfilled, i. e. the density of surface states participating in the nesting must be high enough and the coupling between the electrons and the phonons must be sufficiently strong.

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